Mixing and blending applications, in particular the mixing and blending of liquids, liquid suspensions and gases, are often constrained by the diameter of the tank in which the mixing is being carried out and by the diameter of the impeller. Moreover, the size and diameter of the manway through which the impeller and shaft is inserted can further constrain the mixing application and the impeller employed.
The impeller blades need to be inserted through the manway in the vessel for installation. In some covered mixing vessels, manways are commonly 24″ in size and can pass impeller blades of up to 23″ in width at best. Therefore, in order to insert larger blades, operators either have to install an oversized manway, or the blades must be supplied in a longitudinally split configuration and then assembled inside the vessel. Splitting the impeller blades is an expensive operation, especially for blades having a rounded, leading edge, twist and curvature. In addition, multiple bolts are required along with match marking to assure proper, gap free re-assembly. This process can be very difficult and time consuming because the inner and outer blade components must be aligned correctly so that the impeller balance and blade geometry will not be compromised.
Thus for vessels or containers with relatively small manways, it may be advantageous to utilize a folding impeller as an alternative to splitting the impeller blades or employing an oversized manway. However, conventional folding impellers suffer from a variety of deficiencies. For example, folding impellers must be held well away from the bottom of the container to reduce damage to the container and/or blades in the folded position. Also, the folding mechanism of conventional impellers causes serious disturbances in laminar flow of fluid around the impeller blades. Accordingly, it is desirable to provide an impeller that is capable of overcoming the disadvantages described herein at least to some extent.